As discussed back in June, move the implementation of hmac_sha1 here
because this is not a public function and crypt-sha1.c is the only
place where it is used (It must have been that way formerly, because
there was a stale prototype in crypt.h all the time.)
Being here, do some RCSID and prototype cleanup, and add as much "const"
as needed to compile these files with WARNS=3.